Herschel can see the coldest and dustiest objects in space; for
example, cool cocoons where stars form and dusty galaxies just
starting to bulk up with new stars. The observatory will sift
through star-forming clouds -- the "slow cookers" of star
ingredients -- to trace the path by which potentially life-forming
molecules, such as water, form.

The US space agency NASA played
a key role in the development of two of the mission's three
instruments, and will make important contributions to data and
science analyses. NASA's Jet Propulsion Laboratory in
Pasadena, Calif., developed and built the "spider web" bolometers for Herschel's
spectral and photometric imaging receiver (SPIRE) instrument, which
are 40 times more sensitive than previous versions. It also
developed and built mixers, local oscillator chains and power
amplifiers for the heterodyne instrument for the far infrared
(HIFI).[2]

Instrumentation

The mission, formerly titled the Far Infrared and
Sub-millimetre Telescope (FIRST),
involves the first space observatory to cover the full
far infrared and submillimetre waveband.[3] At
3.5 meters wide, its telescope incorporates the largest mirror ever
deployed in space.[4] The
light is focused onto three instruments with detectors kept at
temperatures below 2 K (−271 °C). The instruments are
cooled with liquid
helium, boiling away in a near vacuum at a temperature of
approximately 1.4 K (−272 °C). The 2,000-litre supply of
helium on board the spacecraft will limit its operational lifetime,
nonetheless it is expected to be operational for at least 3
years.[5]

An imaging camera and low-resolution spectrometer covering wavelengths from 55
to 210 micrometres.
The spectrometer has a spectral resolution between R=1000
and R=5000 and is able to detect signals as weak as −63dB. It operates as an integral field
spectrograph, combining spatial and spectral resolution. The
imaging camera can image simultaneously in two bands (either
60–85/85–130 micrometres and 130–210 micrometres) with a detection
limit of a few millijanskys.[7][8]

Herschel in a clean room

SPIRE (Spectral and Photometric Imaging Receiver)

An imaging camera and low-resolution spectrometer covering 194
to 672 micrometre wavelength. The spectrometer has a resolution
between R=40 and R=1000 at a wavelength of 250 micrometres and is
able to image point sources with brightnesses around
100 millijanskys (mJy) and extended sources with brightnesses
of around 500 mJy.[9] The
imaging camera has three bands, centered at 250, 350 and 500
micrometres, each with 139, 88 and 43 pixels respectively. It
should be able to detect point sources with brightness above
2 mJy and between 4 and 9 mJy for extended sources. A
prototype of the SPIRE imaging camera flew on the BLAST
high-altitude balloon.

HIFI (Heterodyne Instrument for the Far Infrared)

A heterodyne
detector which is able to electronically separate radiation of
different wavelengths, giving a spectral resolution as high as
R=107.[10] The
spectrometer can be operated within two wavelength bands, from 157
to 212 micrometres and from 240 to 625 micrometres.

Service
module

Structurally, the Herschel and Planck SVM's are very similar.
Both SVM's are of octagonal shape and for both, each panel is
dedicated to accommodate a designated set of warm units, while
taking into account the dissipation requirements of the different
warm units, of the instruments as well as the spacecraft.

Furthermore, on both spacecraft a common design for the avionics, the attitude
control and measurement system (ACMS) and the command and data
management system (CDMS), and power subsystem and the tracking,
telemetry and command subsystem (TT&C) has been achieved.

All spacecraft units on the SVM are redundant.

Advertisements

Power
subsystem

On each spacecraft, the power subsystem consists of the solar array, employing
triple-junction solar
cells, a battery and the power control
unit (PCU). It is designed to interface with the 30 sections of
each solar array, provide a regulated 28 V bus, distribute this
power via protected outputs and to handle the battery charging and
discharging.

For Herschel, the solar array is fixed on the bottom part of the
baffle designed to protect the cryostat from the sun. The
three-axis attitude control system maintains this baffle in
direction of the sun. The top part of this baffle is covered with
Optical solar reflector (OSR) mirrors reflecting 98% of the sun
energy, avoiding heating of the cryostat.

Attitude and orbit
control

This function is performed by the attitude control computer
(ACC) which is the platform for the ACMS. It is designed to fulfil
the pointing and slewing requirements of the Herschel and Planck
payload.

The Herschel spacecraft is three-axis
stabilized, the absolute pointing error needs to be less than
3.7 arc sec.

The main sensor of the line of sight in both spacecraft is the
star tracker.

On June 14, 2009, ESA successfully sent the command for the
cryocover to open which will allow the PACS system to see the sky
and transmit images in a few weeks. The lid had to remain closed
until the telescope was well into space to prevent contamination.
Herschel is reported to have completed 90% of the distance to its
orbit 1.5 million km away from Earth.[16]

Five days later the first set of test photos, depicting M51 Group, was published by
ESA.[17]

Operational
mission

On 21 July 2009, Herschel commissioning was declared successful,
allowing the start of the operational phase. A formal handover of
the overall responsibility of Herschel was declared from the
programme manager Thomas Passvogel to the mission manager Johannes
Riedinger.[15]